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The International Celestial Reference Frame (ICRF)

The International Celestial Reference Frame (ICRF) realizes an ideal reference system, the International Celestial Reference System (ICRS), by precise equatorial coordinates of extragalactic radio sources observed in Very Long Baseline Interferometry (VLBI) programmes. The Hipparcos catalogue which includes all the FK5 stars was astrometically aligned to ICRF and provides the primary realization of ICRS at optical wavelengths.